The present invention relates to the field of solenoids and in particular to the type in which the solenoid coil is embedded in some kind of thermoset encapsulation.
It is well known in the art of solenoid manufacture to encapsulate solenoid coils in compounds such as epoxy in order to avoid the adverse effects of various environmental factors and to provide mechanical support and protection against mechanical shock. Solenoids of this type often are exposed to rather hostile environments. An example of such environment is a tractor engine in which the solenoid is used for control of fuel flow. Tractors are often left with their engines running, and they sometimes run out of fuel with the ignition left on. In such a situation, the solenoid remains energized, but is not cooled as it normally is when the engine is operating. As a result, substantial heat can be dissipated that is not effectively removed from the solenoid. Accordingly, the thermoset encapsulation can be subjected to severe thermal stresses, particularly in its thinner portions, that can lead to cracking. Thus, it would appear that relatively thin portions of such encapsulations should be avoided whenever possible.
Another factor in solenoid design is the amount of current or voltage that must be applied to the solenoid in order for it to be activated. Many factors are involved in determining the actuation current, of course, but among them is the reluctance of the magnetic path formed in the solenoid. If the reluctance is relatively low, the magnetic flux produced is relatively high, so actuation current and voltage tend to be reduced by reduced reluctance. One method of reducing reluctance, as is shown in Ludwig, U.S. Pat. No. 3,593,241, is to provide a sleeve or shield made of high-permeability material around the periphery of the coil. Such a shield reduces the reluctance seen by the coil, increasing the flux produced and the force experienced by the armature of the solenoid.
Unfortunately, if it is desired to use such a flux shield in an encapsulated solenoid, the above considerations concerning the thickness of the encapsulation would seem to indicate that provision of a flux shield positioned within the encapsulation would require that the size of the solenoid be increased to prevent further diminution of the encapsulation thickness and thus a greater tendency for the encapsulation to crack.